fix(model): add missing Model interface (Name, Train, Predict, Save, Load)

The registry, ensemble, cv, and tuning packages all reference model.Model
but the type was never defined. This caused build failures when importing
any package that transitively pulled registry or ensemble.

Author: dndungu

model/model.go

@@ -0,0 +1,21 @@
+package model
+
+import "context"
+
+// Model is the core interface for all metee model backends (LightGBM, XGBoost, etc).
+type Model interface {
+	// Name returns a human-readable identifier for this model instance.
+	Name() string
+
+	// Train fits the model on the given feature matrix and target vector.
+	Train(ctx context.Context, features [][]float64, targets []float64) error
+
+	// Predict produces predictions for the given feature matrix.
+	Predict(ctx context.Context, features [][]float64) ([]float64, error)
+
+	// Save serializes the trained model to the given path.
+	Save(ctx context.Context, path string) error
+
+	// Load deserializes a model from the given path.
+	Load(ctx context.Context, path string) error
+}